The invention relates to a workpiece designed for rolling stresses, more particularly to such workpieces fabricated from fully hardening steel and a heat treatment process therefor.
Numerous forms of processes for the heat treatment of steel workpieces are known, such treatment processes generally being used to increase the hardness of the workpieces. The particular type of heat treatment process has a considerable influence on the final properties of the workpieces. As such, the choice of a suitable heat treatment process, in addition to the choice of a suitable steel composition, is of substantial importance to the service life of such workpieces. This is particularly true of workpieces that must satisfy high quality requirements and are exposed to high stresses for a prolonged period of use. In workpieces of this type, the quality may be very significantly dependent on the details of the heat treatment.
DE 101 61 721 B4 has disclosed a heat treatment process for a roller bearing component made from a fully hardening steel, in which first of all carbonitriding is carried out at 860° C. to 930° C. This is followed by austenitizing at 830° C. to 860° C. with a subsequent quench to 190° C. to 230° C. Finally, an isothermal transformation is carried out at between 190° C. and 230° C. followed by cooling to room temperature. The intention of this procedure is for the microstructure transformation in the core and in the surface layer of the component to take different forms and thereby to achieve a retained austenite content of less than 5% by volume in the core and a retained austenite content of more than 10% by volume in the surface layer.
DE 198 49 681 C1 has disclosed a process for the heat treatment of workpieces made from steel or cast iron, in which an austenitization is followed by rapid quenching to approximately the martensite start point. Then, the components are held at the bainite transformation temperature until a desired partial transformation of the microstructure into bainite has taken place. This is followed by rapid cooling to room temperature, brief holding at room temperature and short-time tempering.